1. Field of the Invention
The present invention relates to earth probing systems. More specifically, the invention relates to earth probing systems using low frequency electromagnetic energy to provide detailed information regarding subsurface formations, especially such systems involving system distortion compensation, noise separation, interference removal, and recording, display and mapping of subsurface formations.
2. Related Art
Historically, seismic (sound) waves have been radiated into the earth to detect and map subsurface layers and buried objects. In the recent past, electromagnetic waves have been employed to produce similar detections and mapping. However, results have been limited to shallow ground penetration depths because of the extreme attenuation of the electromagnetic waves as they propagate through the earth. The attenuation of the electromagnetic wave varies drastically with frequency, as seen in TABLE 1.
TABLE 1 ______________________________________ ATTENUATION OF RADAR ENERGY BY A LAYER OF BITUMINOUS COAL AS A FUNCTION OF RADAR FREQUENCY ONE WAY ATTENUATION FREQUENCY (dB per Meter) ______________________________________ 250 KiloHertz 0.005 500 KiloHertz 0.01 1.0 MegaHertz 0.02 5.0 MegaHertz 0.10 25.0 MegaHertz 0.50 100.0 MegaHertz 2.00 500.0 MegaHertz 10.00 1.0 GigaHertz 20.00 ______________________________________
An early example of an attempt to use radio frequency mapping is that of Deardorf (U.S. Pat. No. 1,838,371), who used relatively low power to identify subsurface anomalies in the earth. Potapenko (U.S. Pat. No. 2,139,460) calculated the ratio of absorption of two radio frequencies to identify oil and water deposits. Wheeler (U.S. Pat. No. 2,517,951) employed a wide band antenna, tuned inductively at several points along its length in an effort to transmit more energy into the earth from a radar transmitter. Feder (U.S. Pat. No. 3,351,936) employed two complete radar transmitters and receivers to achieve identification of subsurface reflections. Barret et al. (U.S. Pat. No. 2,901,689) used transmitting and receiving antennas at various angles of incidence and reflection to plot earth cross sections. Chapman (U.S. Pat. No. 4,008,469) employed short pulse, conventional radar equipment along with relatively sophisticated signal processing equipment for subsurface plotting. Fowler et al. (U.S. Pat. No. 4,504,833) uses a synthetic pulse made up of Fourier component frequencies in order to obtain more easily interpreted reflection signals. Thomas (U.S. Pat. No. 5,113,192) employs audio frequency seismic reflection data to help interpret the findings from a ground penetrating radar. Kimura et al. (U.S. Pat. No. 5,130,711) utilizes a polarization-advancing scheme to achieve recognition of signals reflected from underground formations. It is believed that no known systems can achieve penetrations to a depth of a mile, which is frequently required for oil/gas and mineral exploration.
Applicants have recognized that, to obtain electromagnetic wave subsurface maps to depths of one mile or more, the radiated frequency should be about 1-3 Megahertz or less. However, to obtain good resolution, the bandwidth should be as large as possible. To achieve both of the above goals, a large "percentage bandwidth" spectrum must be radiated into the earth. These two conflicting requirements, that of very low operating frequency and a wide percentage bandwidth, represent a challenging design problem for the radar antenna system designer.
As used herein, lower frequency electromagnetic spectrum may be generally divided into bands for the purpose of description, as shown in TABLE 2 (Reference Data For Radio Engineers, Sixth Edition, Howard W. Sams & Co., Inc., page 27-6.):
TABLE 2 ______________________________________ NOMENCLATURE OF ELECTROMAGNETIC FREQUENCY BANDS DESIGNATION FREQUENCY RANGE ______________________________________ ELF--Extremely Low Frequency 3-300 Hz VF--Voice Frequency 300-3,000 Hz VLF--Very Low Radio Frequency 3-30 KHz LF--Low Radio Frequency 30-300 KHz MF--Medium Frequency 300-3,000 KHz ______________________________________
Reception and processing of large-percentage-bandwidth signals using ELF, VF, VLF, LF, and MF electromagnetic waves necessary for earth probing radar systems, require unique instrumentation methods to overcome problems not adequately dealt with by known systems. These problems include proper waveform selection and generation, radiation, reception, and signal processing to detect and enhance subsurface returns. Also, short pulse and linear frequency-modulated pulse compression systems generate spurious signal components due to resonant ringing of the antenna and impedance matching system.
A further design problem has derived from the facts that low frequencies entail long wavelengths, and that antennas which are small compared to the radiated wavelength have very low efficiency and very low gain. Therefore, to use low frequencies would require use of either large antennas or use of high-magnitude currents. Large antennas are impractical, and high-magnitude currents have not been found in systems suitable for high resolution deep-earth probing.
Still another design problem, relating more specifically to signal reception, has derived from the facts that antennas which are small compared to the radiated wavelength have very low efficiency and very low receiving aperture area, and that radar receiving antennas generally display non-uniformity in their frequency response over the large percentage bandwidth required for high resolution earth probing applications. These applications require a high degree of isolation between the transmitting and receiving antennas, to provide adequate sensitivity to the much weaker subsurface return signals. This design problem has not been dealt with adequately in known systems.
Applicants have also recognized that transmission and reception of such signals require unique electrical properties in the antenna system, and special processing techniques for received signals. To achieve sufficient radiated power for ground penetration, a means of maintaining antenna efficiency across the entire operating band must be provided. Reliable detection and location of buried objects by a deep probing radar system further requires precise control and compensation of antenna phase and amplitude response, in order to perform sensitive signal processing operations on the received data.
The present invention embodies unique concepts for solving these and other problems which limit the performance of conventional pulsed radar systems when operating at low frequencies and large percent bandwidths. The novel, heretofore unknown, technology for radar operation in these frequency bands is the subject of the present invention.